1. Field of the Invention
This invention relates to a speaker apparatus.
2. Description of the Related Art
A speaker apparatus in “a moving coil system”, which has been widely known, is shown in FIGS. 1 and 2.
As seen from FIG. 1, a speaker apparatus 100 has a magnetic circuit 104 which includes a yoke 101 equipped with a center pole made of a magnetic material, a ring-shaped magnet 102 arranged on the outer periphery of the yoke 101 and a ring-shaped plate 103 arranged on the magnet 102.
The magnetic circuit 104 has a magnetic gap 104a formed between the inner peripheral surface of the plate 103 and the outer peripheral surface of a protrusion 101a provided at the center of the yoke 101.
Further, the speaker apparatus 100 also includes a cylindrical coil bobbin 105 made of a non-magnetic material and a voice coil 106 attached thereto.
The one end of the coil bobbin 105 is fixedly coupled with the outer periphery of a central hole of a diaphragm 107 having a conical shape. The other end of the coil bobbin 105 is arranged within the magnetic gap 104a. 
The voice coil 106 includes a winding portion 106a where a conductive wire is wound on the outer peripheral surface of the coil bobbin 105 and lead portions 106b where a conductive wires at each of both ends of the winding portion 106a is extracted unidirectionally along the outer peripheral surface of the coil bobbin 105. Each of the lead portions 106b is extracted to the main surface of the diaphragm 107 and connected to an electric supply line 108.
As seen from FIG. 2, in the lead portions 106b of the voice coil 106, the conductive wire at each of at both ends of the winding portion 106a of the voice coil 106 are extracted along the peripheral surface of the coil bobbin 105 in a direction of attaching the diaphragm 107. The lead portions 106b are fixed to the coil bobbin 105 by a winding tape 114 wound on the coil bobbin 105.
Returning to FIG. 1, the central hole of the diaphragm 107 is covered with a cap 109. The diaphragm 107 is attached to a frame 112 through edges 110 and dampers 111 which are made of an elastic material and have an elastic shape.
The frame 112 is provided to a pair of input terminals 113 (only one thereof is illustrated) for supplying an electric signal to the voice coil 106. These pair of input terminals 113 each is connected to the other end of the electric supply line 108 connected to the lead portion 106b. 
In the speaker apparatus 100 having the configuration described above, when a driving current is supplied to the voice coil 106 through the input terminal 113, the voice coil 106 oscillates in an axial direction of the coil bobbin 105 (in the direction of Y—Y line in FIG. 2) owing to the electromagnetic force which is generated between itself and the magnetic field applied by the magnetic circuit 104.
At this time, since the voice coil 106 is coupled with the diaphragm 107 through the coil bobbin 105, the diaphragm 107 also oscillates owing to the oscillation generated in the voice coil 106. Thus, an acoustic energy (voice) according to the driving current is output from the speaker apparatus 100.
Meanwhile, in order that a loud volume of sound is produced from the speaker apparatus 100, it is necessary to supply a large current to the voice coil 106 to oscillate the diaphragm 107 severely. However, if the current supplied to the voice coil 106 is increased, not only the driving force to drive the diaphragm 107 but also the pulling stress and compressive stress applied to the winding portion 106a of the voice coil 106 will be increased.
In this case, owing to the stress generated in the winding portion 106a, the coil bobbin 105 is also subjected to deformation and oscillation etc. As a result, the lead portions 106b fixed to the coil bobbin 105 may suffer from unexpected stress so that it is broken.
In order to overcome such an inconvenience, a proposed method (hereinafter referred to the first method) for preventing the lead portions from breaking is to attach a streamlined wire with a tin (Sn)-plated layer on the surface to the coil bobbin and welding-couple the lead portions extracted from the voice coil with the streamlined wire (refer to JP-A-5-183993).
Another proposed method (hereinafter referred to as the second method) is to cover the lead portions extracted above a damper for supporting the coil bobbin with a sealing member of foamed hot-melt (refer to JP-A-7-015795).
A still another proposed method (hereinafter referred to the third method) is to connect the lead wire extracted from the voice coil to a speaker-dedicated lead wire on the periphery of the coil bobbin using caulking metal fittings (refer to JP-A-2002-125294).